Inkjet recording methods enable simple and inexpensive image production and thus have been used in a variety of printing fields. As an example of the inkjet recording methods, an ultraviolet (UV)-curable inkjet method is known in which droplets of UV-curable inkjet ink are landed on a recording medium and then cured by irradiation with UV rays to form an image. Recently, the UV-curable inkjet method has been attracting attention from the viewpoint that an image having high rubfastness and adhesiveness can be formed even on a recording medium which lacks ink absorbing properties.
There is a demand on an inkjet ink that can be recorded on various recording media other than exclusive paper for inkjet and vinyl chloride for a solvent-based ink. As an ink having a high freedom for selecting a base material, a hot-melt ink and a UV-curable ink are mentioned. Since the hot-melt ink is solid, the ink is supplied in the form of a block shape or a ball shape. The UV-curable ink is directly supplied from an ink cartridge or is introduced to a tank once and then supplied by a pump or the like.
When wax is added to the UV-curable ink, an inkjet ink system is expected which has all of base material selectivity, high-speed printing, and image permanence that are features of each of the hot-melt ink and the UV-curable ink. However, the UV-curable ink added with the wax has a high viscosity and thus cannot be supplied using a liquid sending pump of the related art even at room temperature. For this reason, it is necessary to provide a dedicated facility for sending liquid and thus the facility cost is increased. In addition, if such a UV-curable ink is heated and dissolved in an ink storage unit, the ink storage unit and the whole supply passage including an inkjet head need to be in a heated state, and thus problems arise in terms of safety aspect as well as energy aspect. Several solving means for these problems have been proposed, but the problems have not been solved yet (for example, see Patent Literatures 1 and 2).